Epoxy resins are an important class of thermosetting compounds. They have diverse applications and are widely used in adhesives, structural materials, lacquer, ceramic manufacturing, printed circuit boards, microelectronics packaging, aerospace industry, etc. Epoxy resins usually are hardened or cured by a cross-linking reaction using one of the following three methods. The properties and applications of a cured resin are greatly influenced by the choice of the hardener or the method of curing.
Method 1: An epoxy resin reacts with itself (i.e., homopolymerizes) via a ring-opening polymerization mechanism of the epoxy groups. The self-curing of epoxy resins usually requires an elevated temperature but can be initiated with either a Lewis acid or a Lewis base catalyst (as opposed to a curing agent).
Method 2: An epoxy resin can be cured with a cyclic acid anhydride. The anhydride can react with the epoxy group, pendant hydroxyls, or residual water to form a carboxylate intermediate, which then reacts with the epoxy group, causing a self-perpetuating reaction between the anhydride and the epoxy resin. Catalytic amounts of tertiary amines are commonly used as additives as they facilitate the opening of the anhydride.
Method 3: An epoxy resin reacts with a polyvalent nucleophilic reagent, e.g., a polyamine, at the room temperature. For instance, the ring opening of the epoxy ring with a primary or secondary amine generates a stable C—N bond, which is then cured to form a three dimensional network structure with a high crosslinking density. The epoxy resin can potentially react with potentially every amine group containing an active hydrogen atom, so that, e.g., a simple diamine (NH2—R—NH2) acts as a tetrafunctional cross-linker and reacts with four epoxy groups. Similar to polyamines, polythiol compounds (HS—R—SH) can also react with epoxy groups to form C—S bonds. The reaction of a thiol group with the epoxy group is greatly facilitated by the presence of a catalytic amount of base, such as a tertiary amine, to result in a faster curing process even at the room temperature.
The most common epoxy resin formulations consist of a diepoxide (resin) and a polyamine compound (curing agent or hardener), which form a polymeric network of essentially infinite molecular weight. The combination of “resin” and “curing agent” sometimes is referred to as “after curing (cured) epoxy resin,” “after curing (cured) resin”, or simply “resin” or “epoxy resin.” The widespread utility of such epoxy formulations is due to their excellent processability prior to curing and their excellent post-cure adhesion, mechanical strength, thermal profile, electronic properties, chemical resistance, etc. Furthermore, the high-density, infusible three-dimensional network of epoxies makes it an extremely robust material, resulting in it being the material of choice for many long-term applications. At the same time, this durability makes its removal, recycling and reworkability notoriously difficult, raising concerns about the longevity of epoxy-based materials in the environment. The cross-linking reactions that occur with two convertibly used component epoxies are essentially irreversible. Therefore, the material cannot be easily dissolved, or melted and reshaped without decomposition of the material. The epoxy resin, due to its excellent physical and mechanical properties, electrical insulation, and adhesive performance, is widely used in composite materials, casting parts, electronics, coating, etc. In particular, fiber reinforced epoxy resin composite materials, especially carbon fiber composites, have been widely used in aerospace, automobile, train, ship, wind energy tidal energy, sporting goods and other industries. It has been estimated that by 2015, global composite material production capacity will significantly increase, and exceed 10 million tons. However, how to deal with and recycle the waste of fiber composites has become a worldwide problem that prevents the fiber composite industry's growth, thereby constraining the sustainable development of fiber composites.
By far, the recycling process of fiber composite materials have been roughly reported in the following ways: 1. High temperature thermal degradation (Thermochimica Acta 2007(454): 109-115), which can recycle composite material to obtain clean filler and fiber, but requires high temperature processing and high standard equipment; 2. Fluidized bed (Applied surface science 2008(254): 2588-2593), which requires high temperature processing to recycle and obtain the clean fiber; 3. Supercutical fluid (water (Materials and design 2010(31):999-1002), alcohol (Ind. eng. chem. res. 2010(49): 4535-4541) or carbon dioxide (CN102181071), for degrading epoxy resin system, which is still in the laboratory stage and far from practical industrialization; 4. Use nitric acid (Journal of applied polymer science, 2004 (95): 1912-1916) to degrade the epoxy resin and obtain fiber with clean surface, which has strong corrosion resistance of acid like nitric acid, requires high standard equipment, and results in low operating security, high recycle cost, and difficult post-processing. Generally, these methods have their limitations in varying degrees, existing disadvantages of fiber shortening, performance degradation, environmental pollution, and high recycling cost and so on, therefore, effective and feasible method for the recycling of waste composite materials is still an issue to be addressed in composites field.